Substrate carriers are utilized for holding, transporting and storing substrates, before during and after processing. Examples of such substrates are wafers for use in fabrication of semiconductor devices, magnetic storage disk substrates, substrates for liquid-crystal display panels and the like. Before they can be transformed into the end product, these delicate and valuable substrates are subjected to repeated processing, storage and transport. It is well known that semiconductor wafers are highly susceptible to damage from environmental influences, such as, particulates, electrostatic discharges, jarring and gaseous containments.
Contaminants in the form of dust and other particulates from the ambient atmosphere can irretrievably compromise a substrate should they adhere to it. As the size of an integrated circuitry has continued to shrink, the size of particles which can contaminate an integrated circuit has also become smaller, thereby making minimization of contaminants all the more critical. The semiconductor industry employs elaborate measures such as clean rooms to prevent just such an occurrence. Contamination is accepted as the single biggest cause of yield loss in the semiconductor industry.
Contaminants in the form of particles may be generated by abrasion, such as rubbing or scraping the carrier with the wafers or disks, with the carrier covers or enclosures, with storage racks, with other carriers, or with the processing equipment. A most desirable characteristic of a carrier is therefore a resistance to particle generation upon abrasion, rubbing, or scraping of the plastic molded material. U.S. Pat. No. 5,780,127 discusses various characteristics of plastics which are pertinent to the suitability of such materials for wafer carriers. Said patent is hereby incorporated by reference.
In light of the above, the necessity to protect substrates at each step of the fabrication process becomes obvious. One purpose of a wafer carrier is to provide this protection. An example of a wafer carrier is a Front Opening Unified Pod system (FOUP), such as the F300 Wafer Carrier manufactured by Entegris. A FOUP provides a protective enclosure to shield the wafers from dust particles in the atmosphere or from chemical contamination during fabrication, transfer, transport or storage.
A second purpose of a wafer carrier is to securely hold the wafer disks during transport. Carriers are generally configured to axially arrange the wafers or disks in slots and to support the wafers or disks by or near their peripheral edges. The wafers or disks are conventionally removable from the carriers in a radial direction upwardly or laterally. Commonly assigned U.S. Pat. No. 6,428,729 discloses a wafer carrier configured as a process enhancement carrier provide with wafer support shelves. This patent is hereby incorporated by reference.
Additionally, because processing wafer disks is generally automated, it is necessary for disks to be precisely positioned relative to the processing equipment for the robotic removal and insertion of the wafers. Prior art wafer carriers, such as those utilizing multiple component parts, can have undesirable variations in critical dimensions caused by the stacking of tolerances of the component plastic parts. In order to overcome the difficulty in producing wafer carriers with acceptable tolerances between the wafer planes and external processing equipment interfaces, monolithic wafer carriers have been employed in the art.
The material used for the carrier shell must be such that the carrier maintains its dimensional stability during its product life cycle despite the repeated process and transport induced stresses on the carrier. Dimensional stability is necessary to prevent damage to the wafers or disks and to minimize movement of the wafers or disks within the carrier. The tolerances of the slots holding wafers and disks are typically quite small and any deformation of the carrier can directly damage the highly brittle wafers or can increase the abrasion and thus the particle generation when the wafers or disks are moved into, out of, or within the carrier. Dimensional stability is also extremely important when the carrier is loaded in some direction such as when the carriers are stacked during shipment or when the carriers integrate with processing equipment. The carrier material should also maintain its dimensional stability under elevated temperatures which may be encountered during storage or cleaning.
In handling and processing semiconductor wafers, static electricity is a continuing concern. Conventional carriers used in the semiconductor industry may develop and retain static charges. When a charged plastic part comes into contact with an electronic device or processing equipment, it may discharge in the damaging phenomena of electrostatic discharge (ESD). Therefore, it is desirable to have a carrier with static dissipation characteristics to eliminate ESD as well as to avoid attracting particles. Wafer carriers have been manufactured with conventional static dissipative materials such as carbon filled polyetheretherketone (PEEK) and carbon filled polycarbonate (PC).
Visibility of wafers within closed containers is highly desirable and may be required by end users. Transparent plastics, such as polycarbonates, have been employed for this purpose because such plastics are low in cost. However, they do not have desirable static dissipative characteristics or desirable abrasion resistance.
Carriers are typically formed of injection molded plastics such as polycarbonate (PC), acrylonitrile butadiene styrene (ABS), polypropylene (PP), polyethylene (PE), perfluoroalkoxy (PFA), and polyetheretherketone (PEEK). It must be recognized that a material that is ideal for one carrier function is typically not the ideal material for a different function of the same carrier. For example, PEEK is a material that has ideal abrasion resistance characteristics for wafer contact portions, but is difficult to mold and cost prohibitive relative to other plastics.
Fillers may also be added to injection molded plastics for static dissipation. Such fillers include carbon powder or fiber, metal fibers, metal coated graphite, and organic (amine-based) additives. U.S. Pat. No. 6,428,729 teaches composite substrate carriers, especially wafer carriers formed from at least two different melt processible plastic materials in which the two plastic materials are strategically positioned for optimal performance.
It should be stressed that one of the important considerations in designing a wafer carrier is the cost of the materials used in the construction of the carrier and the ease of molding the material.